RF – Power Amplifier
Class A
NI Multisim 10 tutorial
Design of a Class A amplifier
Open Multisim 10 and start building the amplifier circuit like in figure.
Press CTRL+W to open the Component Library.
Input signalBiasing circuit
RF amplifier stage
LC resonator AND
impedance matching
circuit
Load
Design of a Class A amplifier
Search Component Library for devices you need to build the circuit.
Use P123 transistor in RF-MOS family.
Analog family contains R, C and L
Sources family is for V and I generators
Design of a Class A amplifier
After you placed all components you need to calculate their parameters in order for the amplifier to work properly.
Let’s go though some mathematic.
Design of a Class A amplifier
POWER TRANSFERAssume we want to build a 1W-RF amplifier.
This meas that the power transferred to the load must be 1 W.
So we must guaratee a maximum value of output impedence of 12.5 Ω. Let’s say we use 10 Ω (Rs).
5.1212
5
22
22
max
2
max P
VddR
R
VddP
Design of a Class A amplifier
OUTPUT MATCHING NETWORKWe use a LC “parallel filter” with Q=10.For parallel circuits,
From here, L and C value can be evaluated as:
pHL 8010102
56//
10// XR
XQ
5,5 CLCL XXXXQ
pFC 2,3101052
16//
Design of a Class A amplifier
LARGE INDUCTOR (BFL)BFL inductor works as a RF choke, because it
“chokes off” the flow of RF current through it, and must be “large enough”.
Large enough means at least 10 times the output impedance (Rs).
HBFLRsX BFL
6.110102
10010
6
Design of a Class A amplifier
DC BLOCKING CAPACITORWe must provide a DC blocking capacitor and
an impedance-transforming network. This function may be combined in a LC series circuit with Q=3.4.
Since , evaluate L and C value as
nHR
Qf
RL loadload 235
4.3101022 61
pHRsQf
Cout 470104.310102
1
2
16
X
RQseries
Design of a Class A amplifier
BIAS POINTClass A amplifier bias point must be choosen
in order to have a conduction angle of 360°.
We use R1=3k and R2=10k. This will give us a bias voltage of
VddVV biasgs
VVddRR
RVbias 85,35
13
10
21
2
Design of a Class A amplifier
Double click on a components placed to edit their value.
“Label” sheet allows you to change the component name.
On “Display” sheet you can change view options.
Design of a Class A amplifier
Set signal generator parameters:Voltage (RMS) = 400mVFrequency (F) = 10 MHzLeave other settings unchanged.
Set Vdd to 5V. Do the same for all remaining components
as depicted in schematic.
Design of a Class A amplifier
Now we have designed a circuit like the one in figure.
We can combine the 2 parallel inductors to have a more compact schematic.
Design of a Class A amplifier
nHLL
LLL 60
21
21//
Design of a Class A amplifier
Place “probes” into circuit to measure voltages and currents.
Probe
Design of a Class A amplifier
Reverse output probe direction by right clicking on the probe arrow and choose “reverse” option.
Design of a Class A amplifier
AC ANALISYS Let’s go through the
AC analisys to understand how well we characterized our amplifier.
Design of a Class A amplifier
Set parameters like in figure.
Design of a Class A amplifier
And, again, select output Voltage (should be Probe3) as the variable to analize.
Design of a Class A amplifier
Output graphs show the amplifier is matched to work at 10 MHz. In fact, amplitude response is unitary and phase is 0.
Design of a Class A amplifier
TRANSIENT ANALISYS Now, analyze
amplifier’s behaviour and output signal using transient analysis.
Design of a Class A amplifier
Set up preferences for transient.
Set final time to 2µs Add output variables:
Id (probe 1) Vin (probe 2) Vout (probe 3)
Design of a Class A amplifier
RF – Power Amplifier
Class B
NI Multisim 10 tutorial
Design of a Class B amplifier
BIAS POINTNow, change class A amplifier circuit to build
a new one, but class B. Like for class A, class B amplifier are still linear.
Class B amplifier bias point must be set in order to have a conduction angle of 180°. Changes are in the bias net: Vgs ≈ Vth.
Design of a Class B amplifier
For P123 RF transistor, Vth is about 2V, hence we can change bias circuit resistor’s values to R1=R2=10k.
This means that transistor amplifies only the positive half of the period of the input signal while is off for the rest of time. Complete output waveform is reconstructed by the output LC filter that works as a resonator circuit.
VVddRR
RVbias 5,25
10
10
21
2
Design of a Class B amplifier
AC ANALISYS Again we can analyze
whether the circuit is characterized to work at 10 MHz or not.
Design of a Class B amplifier
You should obtain a frequency response like this one.
As for class A, also class B - RF amplifier is matched to work at 10 MHz.
Design of a Class B amplifier
TRANSIENT ANALISYS
RF – Power Amplifier
Class C
NI Multisim 10 tutorial
Design of a Class C amplifier
Class C are non-linear amplifiers. Bias point for this devices is set to have a conduction angle of less than 180°.AC input generator can be replaced by a
pulse generator. Output waveform still will be a sine due to LC oscillator.
Design of a Class C amplifier
Class C amplifier works with Vgs ≈ 0. This implies a set of new circuit changes:No bias network is required. Input signal voltage must be higher than Vth
in order to be able to turn on transistor.
Design of a Class C amplifier
In order to work this circuit needs to be tuned a little better than class A and B amplifier.
We must change some parametres on output stage in order to improove LC resonator and power efficiency.Cout = 200pFC2 = 4.5nF
Design of a Class C amplifier
Design of a Class C amplifier
Double click on the pulse generator to edit its parameters like in figure.
Design of a Class C amplifier
AC ANALISYS
Design of a Class C amplifier
TRANSIENT ANALISYS
Design of a Class C amplifier
AC behaviuor shows again that amplifier is developed for 10 MHz.
Transient analysis demonstrate that class C amplifier do not amplify voltage since output waveform is quite lower than input. In fact, it amplifies current:Analize input and output current orAnalize input and output power
Design of a Class C amplifier
POWER ANALISYS Use wattmeters to measure input and output power.
Design of a Class C amplifier
Design of a Class C amplifier
Double click on wattmeter to see the power value supplied in that point.
Run simulation to see instant power consumed.
Design of a Class C amplifier
POWER GAIN
EFFICIENCY
842.0101.24
103.203
3
DC
out
P
P
5,45103.446
103.206
3
out
in
P
PG